1. Field of the Invention
The present invention is related to an organic EL (electroluminescence) element, an image display device and a method for manufacturing the organic EL element and the image display device.
2. Description of the Related Art
In recent years, the development of an organic EL element which is a light emitting element capable of emitting light with a high level of brightness by driving it using a DC low voltage is progressing. An organic EL element has a simple structure having two electrodes facing each other, a hole transport layer made of a hole transport material and an organic light emitting layer made of an organic light emitting material, the hole transport layer and the organic light emitting layer arranged between the two electrodes. When an electric current flows between the two electrodes in this organic EL element, the organic light emitting layer emits light and the emitted light passes through a transparent electrode.
In an organic EL element having this structure, both electrodes may be directly formed on both sides of the light emitting layer. However, in order to realize an increase of light emitting efficiency or the like, an injection layer or a transport layer or both is frequently arranged. In particular, a hole injection layer or a hole transport layer or both is arranged between an anode and a light emitting layer. Alternatively, an electron injection layer or an electron transport layer is arranged between a cathode and a light emitting layer. In an organic EL element, the entire structure body including the above plural layers between both electrodes is called a light emitting medium layer.
Types of an organic EL element are divided into an organic EL element using a low molecular organic light emitting material (hereinafter a low molecular organic EL element) and an organic EL element using a high molecular organic light emitting material (hereinafter a high molecular organic EL element).
In a method for forming a low molecular organic EL element, a thin film is generally formed by a dry coating method such as a vacuum deposition method. In a method for forming a low molecular organic EL element, if forming a pattern of a hole transport layer or an organic light emitting layer is required, a layer having a pattern corresponding to an opening part of a mask is formed using a metal mask or the like. However, in such a method for forming a pattern, if the size of a substrate becomes large, it is difficult to obtain accuracy in forming a pattern.
In a method for forming a high molecular organic EL element, the following method for forming a thin film is being attempted. A coating liquid in which an organic light emitting material is dissolved in a solvent is prepared, and the coating liquid is applied to a substrate by a wet coating method. As a wet coating method for forming a thin film, a spin coating method, a bar coating method, a lobe coating method and a dip coating method are well known. However, in the case where these wet coating methods are used, it is difficult to form a pattern of a thin film with a high definition or form a thin film by separately applying three colors of RGB. Therefore, in a method for forming a high molecular organic EL element, it is thought that it is most effective to form a thin film by a printing method which is capable of forming a pattern while a plurality of materials are separately applied.
Further, in an organic EL element or display, a glass substrate is frequently used as a substrate. Therefore, a method using a hard plate such as a printing plate made of a metal as a gravure printing method among various printing methods is not suitable for forming a high molecular organic EL element. On the other hand, an offset printing method using an elastic rubber blanket and a relief printing method using an elastic rubber plate or a light sensitive resin plate are suitable for a method for forming a high molecular organic EL element. In fact, a method by an offset printing method (Patent document 1) and a method by a relief printing method (Patent document 2) are presented.
In the case where an image display device is manufactured, multiple pixels are formed vertically and horizontally and light is emitted, and thereby an image is displayed. Therefore, a light emitting material, a hole injection material or the like is selectively arranged on a pixel electrode, and an organic EL element which is independently arranged for each pixel is formed. In this case, in general, a partition wall which sections respective pixels is arranged beforehand in order that a material is uniformly arranged in respective pixels and light is uniformly emitted.
In a high molecular organic EL element, a hole transport layer is arranged in order that light is emitted by a low applied voltage. A low molecular organic material and a high molecular organic material are usually used for a material of a hole transport layer as a light emitting layer. As a representative example of a low molecular hole transport material, TPD (triphenylamine system derivative: See Patent document 5) is known. As a representative example of a high molecular hole transport material, PEDOT:PSS (a mixture of polythiophene and polystyrene sulfonate: See Patent document 6) is known. A dry coating method and a wet coating method are used for a film formation method as an organic light emitting material.
A convex type structure (Patent document 3) and an inverse tapered structure (Patent document 4) are being examined as a partition wall structure used for a high molecular organic EL element. If a film thickness of a hole transport layer formed on an anode becomes thick, it is difficult for an electric charge to flow. Therefore, non-uniformity in light emitting occurs in an organic EL element which is a current injection type light emitting type. The purpose of the above trial is to control a film thickness of a pixel end (a partition wall end) in a wet coating method. However, in the case where a film thickness of a hole transport layer is thin or conductivity of a film is high, non-uniformity in emitted light does not occur.    Patent document 1: JP-A-2001-93668    Patent document 2: JP-A-2001-155858    Patent document 3: JP-A-2008-210653    Patent document 4: JP-A-2006-048980    Patent document 5: JP-B-2916098    Patent document 6: JP-B-2851185
In the case where conductivity is improved by a film thickness of a hole transport layer or improvement of a hole transport material, problems other than the above non-uniform light emitting occur. In the case where conductivity in a direction of a film thickness of a hole transport layer is called vertical conductivity and conductivity in a direction of a film surface of a hole transport layer (a direction parallel to a surface of a hole transport layer) is called horizontal conductivity, if vertical conductivity is lower than horizontal conductivity, normal light emitting due to an electric field is obtained. However, if conductivity of a film is low, the difference between vertical conductivity and horizontal conductivity becomes small and a current which does not contribute to light emitting (hereinafter called leak current) flows in the direction of a partition wall. A hole moves inside a hole transport layer formed above a partition wall and therefore characteristics of an organic EL element are reduced. In addition, in an image display device including such an organic EL element, if leak current flows between adjacent pixels, it is difficult to control a desirable display.
The present invention was made considering the above problem. A first purpose of the present invention is to provide an organic EL display device and a method for manufacturing the device in which manufacturing is easy and leak current can be reduced or controlled, in an organic EL display device in which a predetermined light emitting medium layer is formed on the entire display region including a part of a partition wall which sections pixels. In addition, a second purpose of the present invention is to provide an organic EL element, an image display device and a method for manufacturing an image display device in which leak current in a direction of a film surface of a hole transport layer is reduced and the element's characteristics are improved in an organic EL element having a hole transport layer.